Fatigue wear: A contribution to the wear phenomena of ion-plated thin metallic films

A wear model based on the fatigue failure of asperities on ion-plated surfaces is presented. It is suitable for ion-plated hard substrates sliding against hard counterfaces where asperity penetrations do not occur. The constants specific to the analysis are evaluated. A wear equation which is dependent on the mechanical properties of the system, the surface topography and the operating conditions is obtained. The wear rates are expressed as functions of the normal load, the sliding speed, the track width, the standard deviation of the surface asperities, the mean radius of the asperities, the stiffness of the asperities and the static yield strength of the multilayered material.     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

The wear of high density polyethylene sliding against steel surfaces

The wear of high density polyethylene sliding against steel surfaces was studied with a pin-on-disc machine. The disc surfaces were finished by a turning operation on a lathe, changing the cutting conditions and tool geometries so as to provide varying surface finishes and different numbers of asperity peaks per unit distance. Optical and scanning electron microscopy were used to study the wear particles and the transfer films. It is found that a polymer film composed of layers about 500–1000 Å thick is formed on the metallic surfaces with sharp asperities. The size of the polymer wear particles decreases with increasing number of asperity peaks per unit distance and with decreasing asperity angle. The steady state wear rate increases rapidly with increasing polymer wear particle size. The wear for polymermetal sliding occurs by the mechanism of abrasion.     

Numerical and finite element contact temperature analysis of real composite-steel surfaces in sliding contact

Numerical techniques have been developed and used to evaluate the contact temperature distribution between real composite-steel surfaces in sliding contact. To characterise the contact temperature problem of composite materials new definitions for composite Peclet numbers have been introduced. In case of `slow sliding' problems a stationary numerical technique was applied, whereas for `intermediate and fast sliding' problems transient finite element (FE) solutions were preferred. At first sliding contacts of a single steel asperity over polyetheretherketone (PEEK) or carbon fibre (CF)/PEEK composite surfaces were modelled in order to study the contact temperature development on a microscopic level. It was followed by contact temperature results for real composite-steel sliding surfaces; the latter helped to provide information about the actual stress conditions, which are necessary to model the wear process of this pair of materials in future works.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

Alumina in unlubricated sliding point, line and plane contacts

The study is based on unlubricated sliding self-mated tests with high-grade alumina in three different contact geometries. In each contact geometry, both mild and severe wear were observed; at the normal force of 30 N that was applied on each test, the transition into severe wear occurred at a velocity specific to the geometry. The wear transition involved surface fracture caused by mechanical and thermal stresses. Part of the wear debris produced under severe wear was compacted under friction and formed smooth tribofilms on the mating surfaces. Larger contact areas allowed slightly higher sliding velocities under a given normal force. The bearing capability of alumina, however, was quite low. Alumina can be recommended only for dry sliding applications in which the load and speed safely remain below the limit for the transition from mild to severe wear.     

Determination of load carrying ability of chemical films developed in sliding point contact

It has already been known for many years that the use of some extreme-pressure (EP), antiwear or friction modifier (FM) additives in mineral oils can produce different kind of boundary or chemical reaction films on sliding contact surfaces of some kinds of steel in boundary lubrication conditions. Using a sliding ball-on-disc configuration lubricated with some kinds of EP or FM, the wear scars on the balls can always reach the same limit size at a specified applied load and sliding velocity. From the fact that the limit sizes of wear scars decrease as sliding speed is increased or applied load is decreased, the load carrying ability of a chemical film can be obtained by extrapolating the data to the condition of zero sliding speed and is so defined that if the contact pressure is greater than this load carrying ability, the contact surfaces will continuously be worn; if the contact pressure is smaller than it, no more wear will occur on the surfaces. Based on this load carrying ability, the hydrodynamic effect of sliding pairs can also be identified. Therefore, the limit size of wear scar at specified sliding speed and applied load can also be predicted in a mixed lubrication condition.     

An investigation of the markings on wear and fatigue fracture surfaces

The worn surfaces of three polymers sliding against a lapped steel disk were studied by scanning electron microscopy (SEM) and transmission electron microscopy. The surfaces of polymethylmethacrylate and high density polyethylene were covered with bands of arced ripples stretched along the transverse direction. The shape of these ripples is consistent with the distribution of tensile principal stress in the contact zone for a hemispherical indenter sliding on a plane surface. The Polyvinylchloride surface suffered severe plastic deformation during sliding as discerned by the dimples on it. The fatigue fracture surfaces of these materials were also examined by SEM. The polymethylmethacrylate surface exhibited a series of striations whose spacing increased in the direction of crack propagation. The striations on high density polyethylene and Polyvinylchloride surfaces were not observed because fracture was accompanied by considerable plastic deformation. This study shows that the mechanism of the separation of a wear particle from the sliding surface is cumulative damage as encountered in fatigue.     

Coating thickness effect on initial wear of nitrogen-doped amorphous carbon in nano-scale sliding contact: Part II—theoretical modeling

This is the second paper of a two-part report. In the first paper, empirical data on the wear particle generation in carbon nitride coatings subjected to repeated sliding contact with a spherical diamond counter-face is reported. The effect of coating thickness on the wear particle generation is also discussed in the first paper. In this paper, a simplified theoretical expression, combining the Coffin-Manson equation with the analytical solution of a proposed elastic perfectly-plastic indentation model, is introduced. The expression successfully correlates critical number of friction cycles for wear particle generation N_c to coating thickness h, contact pressure P and radius of spherical asperity on the tip of the diamond pin R. With this expression, the lifespan of sliding components can be predicted.The theoretical results computed for diamond pin with a specific asperity radius value of 250 nm were compared with the experimental results reported in the first paper. The theoretical model successfully predicts the maximum lifespan of a component, N_c, in repeated sliding applications. The influences of various contact pressures and asperity radii on the maximum lifespan were also assessed using the model.     

Wear and diffusive processes

The paper presents an evaluation of wear produced by adhesion during the contact between asperity of two rough surfaces in relative motion. The numerical results are based on a diffusive model that allows wear volume forming at the tip of a sliding asperity to be evaluated. In turn, the material transfer generated in several diffusive regimes, like normal and anomalous diffusion will be discussed. The quantitative results can facilitate the interpretation of recent experimental features in the field of adhesion and transfer processes.     

Contact and thermal analysis of transfer film covered real composite-steel surfaces in sliding contact

For composite-steel surfaces in sliding contact an anisotropic numerical contact algorithm has been developed to study the ‘layer type’ problems. An FE contact analysis was applied to evaluate the contact parameters (real contact area, contact pressure distribution and normal approach). The contact temperature rise was determined by using both a numerical thermal algorithm for stationary and a FE transient thermal technique for ‘fast sliding’ problems.The effect of a continuous transfer film layer (TFL), that had built up during wear of the PEEK matrix material on the steel counterpart, was considered. Its thickness was assumed to be t=1 μm, and its material properties were that of PEEK at room temperature or, in the case of frictional heating, at a temperature of 150°C (i.e. above the glass transition temperature of the polymer matrix).Results are presented for a spherical steel asperity, with/without TFL, sliding over composite surfaces of different fibre orientation, and in addition, for real composite-steel surfaces (based on measured surface roughness data) in sliding contact. The TFL has an effect on the contact parameters especially at higher operating temperatures (i.e. 150°C); it results in the production of a larger contact area and a lower contact pressure distribution. The contact temperature rise is clearly higher if a TFL is present. Due to the low thermal conductivity of PEEK, the TFL is close to the melting state or it even gets molten within a small vicinity of the contact area.     

A system for wear prediction in lubricated sliding contacts

A system of analysis is developed to predict the rate of wear in sliding contacts. The essence of the approach is the proposal that the rate of wear can be predicted only in probabilistic terms. Therefore, the estimation of the probability of wear, which can be regarded as synonymous with the probability of surface asperity contacts, precedes the calculation of the wear rate. Further, recognising the fact that wear takes place within the actual area of contact, it is argued that this area consists of plastic and elastic contacts between asperities which, in turn, have different shear strengths and contribute differently to the wear process. In the case of lubricated contact, a frictional film defect represents the influence of a lubricant on the wear process. Moreover, as in this type of contact the load is supported by both lubricating film and contacting asperities, a special procedure is provided to estimate the load supported by the asperities, because it is only that part of the load which contributes to the wear. The catastrophic form of wear in lubricated contacts, that is termed ‘scuffing’, is also considered, and the probability of scuffing, under a given set of operating conditions, is estimated. The predictive system has been tested and its predictions are compared with available experimental results.     

Vibration monitoring in sliding wear of plasma sprayed ceramics

Dry frictional contact between two surfaces, one made of plasma sprayed ceramic coatings of Al₂O₃ and Al₂O₃---TiO₂ combination and the other made of steel, is analyzed. The experiments were conducted using a pin-on-disc set-up in the load range of 5–35 N and for sliding distances up to 14 km. The interactions between friction, wear and vibrations due to influence of normal load, sliding speed and system dynamics are investigated in the present paper. Two vibration parameters of pin in the load direction (vertical) are monitored, namely the r.m.s. acceleration and the kurtosis, which seem to be influenced considerably by the wear process and indicate correlation with wear mechanisms taking place such as stick-slip and grain pull-out, as evidenced by scanning electron microscopy of worn surfaces. The study shows that a range of frequency is to be utilized for vibration monitoring to include natural frequencis of the system consisting of pin in contact with disc. This could be estimated by a standard impulse hammer test. The pin acceleration decreases with increase in load and sliding distance, but with respect to sliding speed, the vibration level intially decreases but increases beyond the sliding speed of 1.5 m s⁻¹. Among the three ceramic coatings, TiO₂ is found to be most wear resistant, exhibiting the lowest friction coefficient and a low vibration level. Variation in kurtosis with run-in wear indicates smoothing of Al₂O₃ due to grain pull out.     

Some microscopic aspects of the mild wear of steel*

The paper reviews the experimental classifications of the unlubricated sliding wear of metals, together with some of the suggested relationships between the wear rates, the real area of contact, the load, the hardnesses of the metals and the probability of producing wear particles at the real areas of contact. The microscopic nature of the interactions between sliding surfaces is discussed, with special emphasis on the advantages (and limitations) of using electron microscopy for studying these interactions, and the importance of using X-ray diffraction techniques to complementthe electron microscopy. Examples are then given of how the use of electron microscopy has revealed many facets of the mild wear of steel which would not otherwise have been suspected. For instance, the areas of contact between sliding steel surfaces are comparatively large and few in number, in contradistinction to the situation occurring in stationary contact. These large contact areas crack in a characteristic fashion reminiscent of fatigue failure or brittle fracture. They then flake off at a critical thickness (～ 10^?4cm) to provide the source for the wear debris. The wear debris collects initially in the rough regions which are left behind when the flakes become detached. In this position, the wear debris probably acts in an abrasive fashion. Eventually, the wear debris is thrown clear of the contact area. Analysis of this debris using X-ray diffraction techniques has shown that there is a strong correlation between the presence of certain oxides in the wear debris and the severity or mildness of the wear. It has also been used to estimate the temperatures occurring between the areas in sliding contact. This estimate is consistent with previously published dynamic thermocouple measurements. It also gives rise to consistent values of the activation energy required for oxidation during wear.     

An explanation of the wear of metals

Mathematical analysis establishes that the well-known empirical linear wear law for the adhesive wear of metals is the consequence of the statistics of surface roughness and is almost independent of the assumed contact model. A strain ratio fatigue failure criterion (i.e. the Manson-Coffin low cycle fatigue law) coupled with a probabilistic treatment of surface asperity height distribution and surface contact provides, for the first time, a fundamental explanation for the formation of wear particles.     

A Wear Model of Plane Sliding Pairs Based on Fatigue Contact Analysis of Asperities

Wear modeling is essential to predict and improve wear resistance of machine parts. This article presents a fatigue wear model of plane sliding pairs under dry friction. The wear model is constructed through developing a dynamic contact model of surfaces and proposing a mean fatigue damage constant of asperities. It is simpler and more practical than existing fatigue wear models because it describes the quantitative relationship between the wear behaviors of the plane sliding pairs and the main factors including the load and sliding speed, material property, friction property, and surface topography of the pairs. Furthermore, the wear model can predict the wear of each component of the sliding pairs. Reasonability and applicability of the wear model are validated via pin-on-disc wear tests. The wear model is applicable to predict the wear of the plane sliding pairs, which is characterized by friction fatigue of contact surfaces. The wear model can also be used to guide the tribological design of sliding pairs in machinery.     

Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation

It is shown that the force which opposes the sliding of a hard relatively smooth surface over a softer surface can be explained as the force needed to push waves of plastically deformed material along the soft surface ahead of asperities on the hard surface. For rougher surfaces and/or poorer lubrication it is shown how the wave can be torn off or material removed by a chip formation process and wear particles formed. Coefficients of friction predicted from the corresponding asperity deformation models are shown to give good agreement with experimental results. For smooth well lubricated surfaces the wear of the softer surface is shown to occur as a result of the progressive damage to this surface brought about by the repeated passage of waves across it. Equations for predicting wear are derived from the asperity deformation models and a comparison made between predicted and experimental wear results. The paper ends by considering possible future trends in research into the mechanics of friction and wear.     

Modeling of fretting wear evolution in rough circular contacts in partial slip

This paper presents a numerical model that maps the evolution of contact pressure and surface profile of Hertzian rough contacting bodies in fretting wear under partial slip conditions. The model was used to determine the sliding distance of the contacting surface asperities for one cycle of tangential load. The contact pressure and sliding distance were used with Archard's wear law to determine local wear at each surface asperity. Subsequently, the contact surface profile was updated due to wear. The approach developed in this study allows for implementation of simulated and/or measured real rough surfaces and study the effects of various statistical surface properties on fretting wear. The results from this investigation indicate that an elastic–perfectly plastic material model is superior to a completely elastic material model. Surface roughness of even small magnitudes is a major factor in wear calculations and cannot be neglected.     

Stribeck Curve Under Friction of Copper Samples in the Steady Friction State

The friction and wear of a pure copper block (99.98 wt% Cu) against a hardened steel disc were studied. The effect of sliding velocity and load on the friction coefficient and wear rate of Cu samples during steady tests was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. The transitions from one lubrication region to another are discussed. The mixed EHL region is characterized by stable low values of the friction coefficient, wear rate and temperature. Straight asperity contact is the dominant mechanism under friction of Cu–steel pair in the BL region. High-friction coefficients and wear rates, thin lubricant films and large wear grooves indicate straight asperity contact between rubbed surfaces in the BL region. Although the dominant mechanisms in the mixed EHL and BL regions are different in principle, a steady friction state is preserved in both cases. It is expected that the steady friction state in the BL and mixed EHL regions is associated with deformation and fracture of surface layers but these process occur at different scale levels. It was shown that under friction of Cu–steel pair, two types of ML regions are observed. The first is the stable steady friction of mixed EHL with low values of the friction coefficient and wear rate. The second type of the ML region is the region of unstable friction and wear when a decrease of lubricant film leads to a change of external (roughness, temperature, friction and wear) and internal (strain and stress) parameters. It was found out that a transition to the unstable ML region occurs within a narrow range of Z parameter under definite values of the load and sliding velocity.     

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.